The present invention relates to methods and apparatus for electromagnetic well logging, and more particularly to variable frequency induction well logging employing deconvolution techniques.
In induction well logging, a coil system is lowered into a well bore for the purpose of investigating the electrical properties of earth formations adjacent the well bore. An electrical property of interest in such investigation is the electrical conductivity of particular portions of the formation.
Typically, induction logging coil systems include at least one transmitter coil and one receiver coil, though plural coils or coil arrays are often employed in place of a single transmitter coil or single receiver coil. A time varying signal is impressed on the transmitter coil or coil array and a signal is received by the receiver coil or coil array. The received signals are a function of the coil system structure and the coil system environment, which, of course, includes formation portions of interest.
It is an object of the present invention to approximately determine the true conductivity of incremental horizontal layers of the formation immediately adjacent to the induction coil system. Correlative objects are to minimize or compensate for undesired effects on the measured signal (1) due to propagation effects; (2) due to the presence of nearby layers or strata of arbitrary thicknesses and conductivities; and (3) due to the geometry of the coil system.
It is known in the prior art to measure and record data obtained by an induction coil system at a plurality of depths and combined predetermined fractions of the signals to produce an adjusted value for the conductivity of the formation. Such a technique is taught in U.S. Pat. No. 3,166,709 to Doll. To do this, Doll defines so-called "geometric factors" which are said to depend exclusively on the dimension and position of the coils with respect to the formation. However, the apparent conductivities measured by the induction coil array differ significantly from those estimated by using the Doll geometric factors particularly in high conductivity formations (i.e., greater than 1 mho per meter), as indicated in the article by Thadani and Hall, Jr., "Propagated Geometric Factors In Induction Logging," presented at the 22nd Annual SPWLA Symposium, June 1981.
Conventional induction well logging systems operate at a fixed frequency, for example, 20 KHz. There are at least two disadvantages associated with these systems. First, the dynamic range of the measured apparent conductivity receiver signal is extremely large (0.5 to 2500 mmhos/meter, or approximately 74 db). Second, the vertical propagated geometric response function of the induction coil system (discussed in detail below) is non-linearly dependent on the formation conductivity, and departs significantly from the Doll vertical response function, particularly in highly conductive formations. The deconvolution of fixed frequency induction logs is difficult due to this non-linear variation of the vertical propagated geometric response function.
U.S. Pat. No. 3,119,061 to Tanguy teaches the use of a variable frequency induction well logging system for minimizing the influence of electrical skin effect. Tanguy does not teach the maintaining of the propagated geometric vertical response function approximately constant to facilitate deconvolution. Other patents, of more general interest, discussing the minimization of skin effect, are U.S. Pat. No. 3,147,429 to Moran and U.S. Pat. No. 3,551,797 to Gouilloud, et al.
It is an object of the present invention to provide an induction well logging system which obtains accurate conductivity measurements for incremental formation layers through wide variations in formation conductivity.
It is a further object of the present invention to provide an induction well logging system which obtain accurate conductivity measurements through wide variations in formation conductivity, without requiring instrumentation with wide dynamic range.
The aforementioned Doll technique does not provide a rigorous means for determining true conductivity and delineating accurately layers of selected thickness for the useful, general case of a cylindrically symmetric formation having an arbitrary arrangement of layers of strata of different, arbitrary thicknesses and conductivities because propagation effects are ignored.
It is an object of the present invention to provide such a technique for use with a logging system which obtains accurate conductivity measurements through a wide range of formation conductivity.
These and other objects and features of the present invention will become apparent from the claims and from the following description when read in conjunction with the accompanying drawings.